This invention relates to photoresists suitable for the manufacture of printed circuit boards and the like, and more specifically solid photoresists as contrasted to liquid photoresists.
Photoresists are light sensitive materials that can be coated or bonded to substrates, and can be differentially removed in the form of a predetermined pattern. The remaining material may act as a resistant material to chemical or physical action applied to the substrate. Liquid photosensitive resists have been used since the 19th century for printing plates and for printed circuits since their inception in the 1940s.
Liquid photoresists are applied by dipping, rolling, whirling and other methods which have several disadvantages. Some of these include drying times and clogging of holes in printed circuit boards.
In recent years presensitized solid resists in the form of a laminated 3 layer film have become popular. The photoresists is quite fragile and requires a plastic sheet on each side of the resist film for protection. A process for preparing such a photoresist is described in U.S. Pat. No. 3,469,982.
During use, one outer layer is removed as the film is laminated under elevated temperature to a substrate. In printed circuits, the substrate would be the copper surface of a copper clad insulating laminate. The other outer plastic layer of a clear plastic remains on the photoresist layer during the exposure to prevent the photostencil from adhering to the photoresist and also possibly damaging it.
The clear plastic layer prevents the photostencil from intimate contact with the photoresist. Any separation degrades the sharpness of the image in contact resist printing, so that the clear plastic layer degrades the image quality in proportion to its thickness.
A problem with such a presensitized film is that it can be used only in a subdued light area. Another disadvantage is the limited storage life of the presensitized materials. Yet another is the high cost of producing such a specialized resist.
In spite of the shortcomings of the solid presensitized resists, they have several advantages over the liquid resists, and are rapidly gaining in usage.
It is an object of the present invention to provide a new and improved photoresist and process for making solid photoresists. Other objects, advantages, features and results will more fully appear from the following description.